Method of removing water of gelation



Patented Jan. 21, 1936 Milton J. Shoemaker, Madison, Wis alsignor, bymesne assignments, to Carbide and Carbon Chemicals Corporatio porationof New York Application November 24, 1931. serial N0. 577,159

N Drawing.

TCIaims.

This invention relates to a metflod for removing water rapidly andefliciently by a freezing process from solid cellulosic products whichhave been coagulated, regenerated or otherwise de- I posited fromaqueous solutionsand especially from such products as freshly formedsheets of regenerated cellulose (as from viscose) and of alphylhydroxy-ethers of cellulose such as ethyl hydroxy-ethers which have alimited solubility in caustic alkalies and to the product resultingtherefrom. Some of the objects of this invention are to cheapen the costof drying films, threads or the like, of celluloslc products which havebeen formed from water or water solutions, to cut.

the cost of the machinery necessary for drying, to simplify the dryingoperation by decreasing the shrinkage on drying, and to increase thestrength of the materi 4 At the present time films of regeneratedcellulose are 'made by coagulating a film of viscose in an acidifiedbath after which the film is led through a number of treating andwashing baths.

a when the film is a continuous sheet, it finally 88 over a series ofheated drums to dry the product to the desired moisture content. Thislatter operation must becarried out carefully on equipmentwhichordinarily is expensive, to produce a smooth, highly transparent,and lus- '80 trous article, such as is demanded by the trade.

The evaporating costs are high 'since the wet sheet enters the dryingmachine with a water content of 75% to 90% (if no emollient is used) andthis water must be practically entirely re- 85 moved. If sheets of.certain derivatives of cellulose, such as the hydroxy-ethers aspreviously described, are made; approximately the same conditionsobtain. During the evaporation of this large amount of water the sheet,if allowed to 40 contract freely, is subject to excessive shrinkage,

the average lineal shrinka e being about 30 to 35 per cent. Thiscomplicates the drying prob? lems. However, when a sheet is dried oncontinuous machines it is kept under tension so that there is little orno lineal shrinkage but there is some cross shrinkage. As a result thesheet is stretched, usually'more or-less unevenly, and there isconsiderable variation in the thickness and in the strength of the sheetboth across and 60. in the machine direction. By my invention it ispossible to eliminate over per cent of the evaporation cost andmachinery for making a continuous sheet and simultaneously, cut thetendencytoshrinkduiingdryingtoaminimum.-

This lessened tendency to greatly dcn, New York, N. Y., a 1'? creasesthestretchinthedriedsheetandmakes for more uniformity in thickness andin the strengthof the shectbothacross andinthemachine direction. Inaddition it is not necessary to handle-on the'driers, the very tenderfilm 5 having a high water content. 'When .the before mentionedcellulosic products are coagulated from a'water solution, and afterbeing further treated and/or washed to remove impurities in the form ofother products of re- 10 action and coagulating acids and salts, thecoag- I ulated material contains from 75% to 90% of water. This watermay be described as water of gelation. All of this water of gelation,however, apparently is not held in the same way by the 15 coagulatedcellulosic material. If about to of the water present in this coagulatedmaterial (containing about to of water) is removed. water will not againreenter appreciabLv the partially dehydrated film (con- 2 taining 60% to70% of water) even on prolongedsoakinginwater. Thebalanceofthe water maybe removed by drying but the dried material will again take up a likeamount of water (60% to 70% of water) if soaked in 25 water. The firstdehydration or de-gelation, therefore, is an irreversible phenomenomandI have called the water which is removed in this step non-reentrantwater of gelation since such water cannot again enter the material afterbeing 30 removed therefrom.

I have found that it is possible to remove'the non-reentrant water ofgelation by a freezing operation. If the freshly coagulated and washedmaterial, for example, in the form of a film, is 35 frozen and thenthawed, water drains from the surface thereof, About 55% to 60% of thewater content of the film is removed by this procedure. The linealshrinkage during this operation is about 6% for the hydroxy-ether. Thepresence 40 ofan emollient such as slycerine reducesthis shrinkage.The-balance of the drying may be accomplished in'the usual way, theadditional shrinkage beinxabout 8%. After thefilm is .frozen and thawed,and the non-reentrant water 5 removed asby draining, the-strength of thefilm is increased considerably over that of the initially coaculatedfilm and its thickness is reduced. An emollient such as glycerine, ispreferably introduced into the film after the non -re- 50 ventrant waterhas been removed. The film is run through the necessary bath ofemollient in the usual manner. The freezing operation may be carried outby placing the film against a cold surface or sub- 55 of ordinary. salt(sodium If thefrolennlm-remains 2 iecting it to cold air or to anysuitableeold liquid. The film, if it is to be frozen in the-air or anyother gaseous fluid. is subjected to such g'as cooled to a temperaturebelow the freezing point of, waters The rate of freezing depends uponthe temperature and circulation of the air. If the gas circulatesrapidly it freezes the film more rapidly, but it also causes cloudinessand for most this is objectionable.

An excellent freezing method is that inwhich the coagulated cellulosicmaterial is immersed in a liquid bath which is held at a temp raturebelow the freezing ,point of water. Obviously the bath should be of sucha materialthat it does not affect the dim chemically. Mercury isespecially suitable. Although various organic liquids such as toluene,gasoline, alcohols, carbon tetrachloride, be

sufficient quantity which permit the solution to lee-cooled considerablybelow the freezing point ofwatermaybeusedtoadtagesincethefllms frozentherein remain clear and bright. solutions chloride) or calcium chlorideare suitable. The bath is cooled to a tem-' perature from 5 to 15degrees centigrade belowi the freezing point of water, and theeoagulated and washed film immersed therein. The ordinary fllm freezesquickly, ice forming a coating over the, surface and extending throughit in 10 to 20 seconds. The film may now be removed from the cold-bathto awarm'er place, to allow the ice to melt, or the film may be allowedto remain in the bath for -a longer period of time. in a refriaeratlnzbath of a solution-of a salt inwater for a period of timethe melts andthe water resulting therefrom gradually diffuses into the refrls ratingsolution sothat'thefllmthenmaybera moved with the ice completelyliquefied. After be carried out in definitive action of liquids'coagulatc from takes place by conversion of that portion, from water ofg'elation to frozen non-reentrant water. The removal of the frozennon-reentrant water is not necessarily practiced by melting, and maynumerous ways. Removal has been described by melting it, and also bydissolving it, as it were, by the diffusive or other thereon which areeffective below the melting point of the ice, such as a salt solution inwhich the ice may be formed.

Other known methods of removing ice may be applied, such as evaporationof the ice without melting it.

It is also to-be understood that the invention may be practiced onmaterials from which part of'the waterhas already been dried, but inwhich a portion of the non-reentrant water yet remains. In commercialpractice of the invention, the methods employed for storing, conveyins,or exposins the material after coagulafreezins may be intentionally orinadvertently conducive to dryina, and may effect partial drying. Suchpractice does not in any way mitigate the advantagesof the presentinvention nor depart therefrom.

While cellulose regenerated from viscose, and the hydroxy ethers ofcellulose have been speciiically described, it will be understood thatthe functional property 'of hydration upon coasulation from water iscommon to cellulose and its derivatives, including the hydroxy ethers,

v conteniplatesallsuch materials which are charaeteriaed by this pr prty.

It will be understood that numerous chang in and departures from theprocesses particularlydiscloeedhereinmaybemadewithoutdepartingfromthespirltand tionasinthe appended claims.

Iclaim;

1. 'lhe'methodofsubstantially decreasingthewatercontentofacellulosicfilmwhichhssbeen aqueous solution-and whichcontains non-reentrant water of gelation,

prisuimmersinssaidiilniinanaqueousresolution of a salt which is at aperature below the freezing point of water,

non reentrant water of 'freeses'and into-said refrigeratinssolufrom'sfld liqui scope of the inven been coagulated from aqueuossolution and which contains non-reentrant water of gelation, whichcomprises freezing said coagulation product in a solution, and removingthe ice formed by said freezing process by the results ofdiflusion'between the solution and the product containing the icecrystals at a temperature which is below the freezingpoint of said ice.

4. As a new article of manufacture, a. watermoist film of a hydratedcellulosic product from which non-reentrant water of gelation has beenremoved, said film being characterized by uniform tensile strength inright-angular directions in the film.

5. As a new article of manufacture, a watermoist film of a hydratedcellulosic product from Y which non-reentrant water of gelation has beenremoved, said fllm being characterized by uniform shrinkage and tensilestrength in rightangular directions in the film.

6. As a new article of manufacture, a watermoist film of regeneratedcellulose from which non-reentrant water of gelation has been removed,said film. being characterized by the property of uniform shrinkage andtensile strength in right angular directions in the film.

'7. As a. new article of manufactures. watermolst fllm of alphyl-hydroxyether of cellulose from which non-reentrant water of gelation has beenremoved, said film being characterized by the property of uniformshrinkage and tensile strength in right angular directions in the film.15

MILTON J. SHOEMAKER.

